The present invention relates to removal of vanadium from phosphoric acid and, more particularly, to a process for removal of vanadium from a wet process phosphoric acid by ion exchange.
Phosphoric acid is produced in large quantities by chemical companies all over the world. The raw material feed for phosphoric acid production is typically the mineral fluorapatite, a calcium phosphate fluoride compound. Digestion of this mineral in an aqueous solution of a strong acid such as sulfuric acid solubilizes the phosphate species. The product slurry is then filtered to yield a liquid usually containing about 28-30% P.sub.2 O.sub.5. In addition to phosphorus, the acid contains soluble impurities that were present in the raw feed and which dissolve in the 28-30% P.sub.2 O.sub.5 liquid. Among these impurities is vanadium at concentrations that usually range from 50 to 1500 ppm, as V.sub.2 O.sub.5, and even higher.
When present at high enough levels, vanadium is toxic to animals. Quite clearly, therefore, the presence of vanadium in phosphoric acid destined for processing into agricultural products such as animal feeds is to be avoided. In fact, the toxicity of vanadium is such that if it is present at levels greater than approximately 250 ppm, as V.sub.2 O.sub.5, in a 30% phosphoric acid, the downstream products will be unusable for animal feed. It is also desirable to remove vanadium from a phosphoric acid in view of its commercially valuable applications. More specifically, it is well known that vanadium has an economic value as a strategic metal. Additionally, vanadium is of great importance to the steel industry which employs vanadium in the form of ferrovanadium alloys. Vanadium pentoxide granules are also utilized as a catalyst in the production of sulfuric acid from sulfur dioxide gas feed.
Thus, both in terms of its deleterious effect on agricultural products formulated from phosphoric acid and in terms of its value in its own right in a number of commercial applications, it is desirable to remove vanadium from phosphoric acid and to recover such vanadium for separate commercial use.
The desirability of removing vanadium from phosphoric acid notwithstanding, the art has thus far failed to provide a reasonable process for doing so. To the contrary, the major processes available for vanadium recovery namely, direct precipitation and solvent extraction, both suffer from a number of significant shortcomings which are discussed in greater detail below.
For example, Waggerman describes, in Phosphoric Acid, Phosphates and Phosphatic Fertilizers, 2d. edition, Rheinhold Publishing Corporation, pp. 189-190 and 230-231 (1952), a process for recovery of vanadium from phosphate deposits as a by-product in the manufacture of phosphoric acid by either the thermal reduction method or the sulfuric acid process. In the sulfuric acid process (which is the same as the wet phosphoric acid process), the vanadium which is dissolved in sulfuric acid is recovered by the addition of sodium ferrocyanide which causes it to precipitate. Precipitation is achieved alternatively by concentrating the acid to 53.degree. Beby evaporation, filtering the acid, and then adding sodium chlorate.
A process involving precipitation of vanadium using oxidizing agents is described in U.S. Pat. No. 2,193,092 by Frick et al wherein phosphate rocks containing vanadium are leached with sulfuric acid for the preparation of phosphoric acid. When the impure phosphoric acid is evaporated to 53.degree. Be and treated under suitable conditions with an oxidizing agent such as sodium chlorate, the tetravalent vanadium is oxidized to the pentavalent oxide form. The pentavalent vanadium then combines with phosphoric acid to form hydrates which are relatively insoluble in water at ordinary atmospheric temperatures. These hydrates, phosphate vanadic acids, are yellow crystalline compounds readily separated from the phosphoric acid by settlement and filtration. The precipitate may then be purified by heating it with a solution of soda ash and hydrated lime. The calcium reacts with the phosphorus producing substantially insoluble calcium phosphate and liberating vanadic acid, with the production of a vanadic acid solution and an insoluble residue.
The recovery of vanadium from phosphoric acid solutions by a precipitation technique as described above gives rise to a number of disadvantages. In the first place, the above-described sodium ferrocyanide precipitation process requires the addition to the phosphoric acid solution of a precipitating agent. Such agent, which is a chemical species which is foreign to the phosphoric acid mixture, can be deleterious to the quality of the final product and, consequently, to the product's marketability. Another problem of a precipitation technique relates to the fact it always leaves a residual quantity of vanadium in solution that corresponds to the solubility of V.sup.+5. As a consequence, the amount of vanadium ultimately recovered is diminished by the amount of V.sup.+5 which remained in the phosphoric acid solution.
As an alternative to a precipitation process, solvent extraction processes have also been developed for the removal of vanadium from phosphoric acid solutions. For example, tri-isooctyl phosphine oxide (TOPO) has been demonstrated to extract vanadium from wet process phosphoric acid. Specifically, as set forth in Koerner et al, U.S. Pat. No. 3,700,415, vanadium is recovered from wet process phosphoric acid by extraction of the acid at a pH of from about 0.0 to about 1.5 using an organic extractant comprising a hydrocarbon solvent and a neutral organic phosphorus compound, for example, tri-n-octyl phosphine oxide.
U.S. Pat. No. 3,374,696 of Lucid et al also describes a solvent extraction process for recovery of vanadium from acidic solutions containing fluoride using amines in a water-immiscible organic solvent. The vanadium is complexed with the extractant and extracted into the organic phase. Prior to the extraction, wet process phosphoric acid is treated with an oxidizing agent to convert the vanadium from the +4 to the +5 oxidation state. Without converting the vanadium to the pentavalent state, the extraction process with the complexing agents is not satisfactory, as vanadium in the lower oxidation state does not form a complex with the extractants contemplated within the scope of the invention. Fluoride, which is believed to complex with the vanadium and the complexing agent, must be present in the wet process phosphoric acid in order for the extractants to properly complex the vanadium.
The above-described solvent extraction techniques, as with the precipitation techniques, give rise to a number of substantial disadvantages. In the first place, solvent extraction processes are accompanied by organic entrainment into the aqueous phosphoric acid phase and its detrimental consequences. Such organic compounds act as solvents and can cause damage to downstream processing equipment that contains rubber and plastics. Additionally, many of the organics employed are themselves toxic. Finally, solvent exchange processes virtually invariably give rise to the formation of a third phase of emulsified crud. Such crud is particularly undesirable in large scale processes since it reduces the capacity of settlers, interferes with mass transfer, and leads to losses of both the organic and the aqueous phase materials from which the crud is composed. While technology does exist for the removal of the valuable components from the crud, it is clearly preferable if formation of such crud can be avoided from the outset in order to avoid the time and costs which would be associated with the additional processing.
Thus, in terms of the amount of product recovered, the purity of the product recovered, efficiency of the recovery process itself, and deleterious effects on the treated acid, prior art processes have proven to be unsatisfactory in separating vanadium from a wet process phosphoric acid.